The present invention relates to a circuit for switching microwave energy between alternative paths.
SPDT microwave switches are currently realized in monolithic form by using resonated field effect transistors connected in either series or shunt with the output transmission line. For the case of the series configuration, isolation in the "off" path of the switch is realized by biasing the gate to pinch-off the channel. At high microwave frequencies, the drain-source capacitance shunting the high resistance state of the FET severely degrades the isolation values obtainable with the switch. By use of an inductive element to parallel resonate the drain-source capacitance, suitable values of isolation can be obtained, but only over a narrow frequency band.
A configuration utilizing shunt connected FETS has been realized by spacing the devices one-quarter wavelength away from the junction. In this configuration, due to the impedance transformation (phase shift) effected by the quarter-wavelength line, the circuit branch is "on" when the corresponding device is "off", and vice versa. The devices are again parallel resonated to obtain low loss in the "on" state. Broader isolation bandwidth is achievable with this configuration because the "off" branch is realized with the FET in the inherently wideband, low resistance "on" state. However, the shunt configuration requires nearly twice the area of the series switch, a distinct disadvantage in most monolithic applications.